Working on fitness before the season approaches is important, and one way is FARTLEK training. The Swedish word for "speed play" speaks for itself, it is an opportunity to work on speed and fitness and would be a good approach for football where you are constantly on the move like Fartlek training suggests. A typical training model is attached, but it can literally be made structured e.g 1minute jog, 1 minute hard running, 2 minute jog, 2 minute hard running etc, but your cycle should be continuous until you need a rest then begin again.